The invention relates in general to the field of microfluidic devices and methods of fabrication and operation thereof. In particular, it is directed to microfluidic devices equipped with microvalves.
Microfluidics generally refers to microfabricated devices, which are used for pumping, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale (see, e.g., Brody, J. P., Yager, P., Goldstein, R. E. and Austin, R. H., 1996 Biotechnology at low Reynolds Numbers, Biophys. J. 71, 3430-3441, and Knight, J. B., Vishwanath, A., Brody, J. P. and Austin, R. H., 1998 Hydrodynamic Focusing on a Silicon Chip: Mixing Nanoliter in Microseconds, Phys. Rev. Lett. 80, 3863-3866). Flow of liquids in microfluidics is typically laminar. Volumes well below one nanoliter (nL) can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions that are limited at large scales (by diffusion of reactants) can be accelerated (see Squires, T. M. and Quake, S. R., 2005 Microfluidics: Fluid Physics at the Nanoliter Scale, Rev. Mod. Phys. 77, 977-1026). Finally, parallel streams of liquids can possibly be accurately and reproducibility controlled, allowing for chemical reactions and gradients to be made at liquid/liquid and liquid/solid interfaces (Kenis, P. J. A., Ismagilov, R. F. and Whitesides, G. M., 1999 Microfabrication Inside Capillaries Using Multiphase Laminar Flow Patterning, Science 285, 83-85). Microfluidics are accordingly used for various applications in life sciences.
Many microfluidic devices have user chip interfaces and closed flowpaths. Closed flowpaths facilitate the integration of functional elements (e.g., heaters, mixers, pumps, UV detector, valves, etc.) into one device while minimizing problems related to leaks and evaporation.
The analysis of liquid samples often requires a series of steps (e.g., filtration, dissolution of reagents, heating, washing, reading of signal, etc.). For portable diagnostic devices, this requires accurate flow control using various pumping and valve principles. It is usually a challenge to obtain valves that are simple, inexpensive to fabricate and easy to operate.
Two categories of valves for microfluidic devices (or “microvalves”) can generally be identified: (i) the active valves and (ii) the passive valves.
Active microvalves usually have increased fabrication complexity, are expensive to fabricate, and need power for actuation. They further need external peripheral and also need power to stay in “on” or “off” state. An example is the “abrupt junction passive microvalve”. Such a microvalve requires active pumping to pump aqueous liquids inside hydrophobic structures, where they can be pinned at constriction. Increasing the pumping pressure results in pushing liquid through the valve. As it may be realized, such a solution is however not compatible with capillary-driven microfluidics. It further requires active pumping and actuation, i.e., additional peripherals. In addition, liquids tend to break in larger volume before a constriction.
Next, passive microvalves usually lack interactivity (i.e. they impose predefined opening or closing conditions), require complex fabrication of integration of chemicals. In addition, passive valves that are initially in closed state usually have problems with venting of air.
The following references address various types of microvalves that have been developed so far:    Liu, et al. Anal. Chem. 2004, 76, 1824-1831.    Ahn, et al. Proc. of the IEEE, Vol. 92, No. 1, January 2004, pp. 154-173.    Zoval, et al. Proc. of the IEEE, Vol. 92, No. 1, January 2004, pp. 140-153.